1. Field of the Invention
The present invention relates to an electrophotographic photoconductor in which a bisamine compound having a specific bisbutadiene structure is included in a photosensitive layer formed on a conductive support, and an image forming apparatus.
2. Description of the Related Art
In recent years, organic photoconductive materials are pit on wide-ranging studies and researches. The organic photoconductive materials not only are used for electrophotographic photoconductors (hereinafter, also referred to as simply “photoconductor”), but also have become used in applications of electrostatic recording devices, sensor materials or organic electro luminescent (abbreviation: EL) devices. Further, the electrophotographic photoconductor using the organic photoconductive material is used not only for the copier field, but also for the fields of printing plates, slide films and micro films, where a photographic technology is heretofore used, and is also applied to high-speed printers using laser, a light emitting diode (abbreviation: LED) or a cathode ray tube (abbreviation: CRT) as a light source. Therefore, requirements of the organic photoconductive material and the electrophotographic photoconductor using the organic photoconductive material are becoming highly advanced and widely spread.
Hitherto, as the electrophotographic photoconductor, widely used are inorganic photoconductors which are provided with a photosensitive layer containing, as its major component, an inorganic photoconductive material such as selen, zinc oxide or cadmium. The inorganic photoconductor has fundamental characteristics required as photoconductors to some extent. However, the inorganic photoconductor has a drawback such that it has a difficulty in forming a film of the photosensitive layer, is deteriorated in plasticity and has high production cost. Also, the inorganic photoconductive materials are highly toxic in general and are therefore largely limited in its production and handling.
On the other hand, an organic photoconductor using the organic photoconductive material has advantages that a film formation property of the photoconductive layer is good, flexibility is high, and the photoconductor is lightweight and highly transparent, and a photoconductor, which exhibits a good sensitivity for a wide-ranging wavelength region by an appropriate sensitizing method, can be easily designed, and therefore the organic photoconductor is being developed as mainstreams of the electrophotographic photoconductor. The organic photoconductor had defects in a sensitivity and a durability early, but these defects are outstandingly improved by a development of a function separated electrophotographic photoconductor in which a charge generation function and a charge transport function are shared on separate materials respectively. The function separated electrophotographic photoconductor also has an advantage that each of a charge generation material bearing the charge generation function and a charge transport material bearing the charge transport function can be selected from wide-ranging materials and a electrophotographic photoconductor having any characteristic can be relatively easily prepared.
As the charge generation material to be used in these function separation type photoconductors, studies have been made as to various materials such as a phthalocyanine pigment, a squalilium dye, an azo pigment, a perylene pigment, a polycyclic quinone pigment, a cyanine dye, a squaric acid dye and a pyrylium salt-based dye, and a variety of materials having a high light fastness and a high charge generation ability have been proposed.
On the other hand, as the charge transport material, there are known various compounds such as pyrazoline compounds (e.g., see Japanese Examined Patent Publication No. 52-4188), hydrazone compounds (e.g., see Japanese Unexamined Patent Publication No. 54-150128, Japanese Examined Patent Publication No. 55-42380 and Japanese Unexamined Patent Publication No. 55-52063), triphenylamine compounds (e.g., see Japanese Examined Patent Publication No. 58-32372 and Japanese Unexamined Patent Publication No. 2-190862) and stilbene compounds (e.g., see Japanese Unexamined Patent Publication No. 54-151955 and Japanese Unexamined Patent Publication No. 58-198043). In recent years, pyrene derivatives, naphthalene derivatives and terphenyl derivatives (e.g., see Japanese Unexamined Patent Publication No. 7-48324), which have condensed polycyclic hydrocarbons as a central mother nucleus, are developed.
The following items of the charge transport material are required:
(1) to be stable against light and heat;
(2) to be stable against ozone, nitrogen oxide (NOx) and nitric acid respectively generated by corona discharge in charging a surface of the photoconductor;
(3) to have a high charge transport ability;
(4) to have a high compatibility with an organic solvent or agent; and
(5) to be easily and economically produced.
However, the above-mentioned charge transport materials satisfy a part of these requirements but do not satisfy all of the requirements at a high level.
Further, among these requirements, it is particularly required to have a high charge transport ability. For example, when a charge transport layer formed by dispersing the charge transport material together with a binder resin is a surface layer of the photoconductor, the charge transport material requires the high charge transport ability in order to secure a sufficient photoresponsivity. When the photoconductor is disposed in a copier or a laser beam printer for use, the surface layer of the photoconductor is necessarily partly shaved off by a contact member such as a cleaning blade or a charge roller. In order to enhance a durability of the copier or the laser beam printer, a surface layer resistant to these contact members, that is, a surface layer having a high printing durability, which is less shaved by these contact member, is required. Thus, if a content of the binder resin in the charge transport layer, the surface layer, is increased in order to strengthen the surface layer and improve the durability, the photoresponsivity is deteriorated. A reason for this is that since the charge transport ability of the charge transport material is low, the charge transport material in the charge transport layer is diluted as the content of the binder resin increases, and therefore the charge transport ability of the charge transport layer is further lowered to deteriorate the photoresponsivity. When the photoresponsivity is poor, a residual potential is increased and the photoconductor is used repeatedly with a surface potential of the photoconductor not adequately decreased, and therefore a surface charge of a part to be eliminated by exposure is not adequately eliminated and this causes troubles such as reduction in image quality at an early stage. Accordingly, in order to secure the sufficient photoresponsivity, the high charge transport ability of the charge transport material is required. Further, a material, which can secure an adequate printing durability while maintaining the high charge transport ability without increasing a ratio of the binder resin by firmly bonding to the binder resin, is required.
Moreover, recently, downsizing and speeding up of electrophotography apparatuses such as a digital copier and a printer are advanced, an increase in sensitivity according to speeding up is required as the photoconductor characteristics and a higher charge transport ability of the charge transport material is required. In a high-speed process, since a time between exposure and developing is short, a photoconductor having a high photoresponsivity is necessary. As described above, since the photoresponsivity depends on the charge transport ability of the charge transport material, also from this point of view, the charge transport material having a higher charge transport ability is required.
With respect to a charge transport material satisfying such requirements, as a compound having a higher charge mobility than the aforementioned charge transport material, a trial of a compound, in which a charge mobility of the charge transport material is increased by having a bisbutadiene structure in a molecule to expand a conjugated system in the molecule, is made (e.g., Japanese Unexamined Patent Publication No. 2002-275135 and Japanese Unexamined Patent Publication No. 9-244278).
However, even though various organic compounds are developed as the charge transport material like these, there is no organic compound overcoming all of the following problems such as:
1) a compatibility with a binding agent is low;
2) crystal is easily precipitated;
3) changes in sensitivity occur in the case of repetitive use;
4) a charging ability and a repetitive characteristic are low; and
5) a residual potential characteristic is poor.
It is a current status that an organic compound, which satisfies essential properties required as the photoconductor previously described, a mechanical strength and a high durability, is not yet adequately attained.